The present invention relates to a magnetic filter apparatus for continuously separating magnetic particles contained in fluids, which is used in cleaning treatment of various types of fluid such as rolling oil for cold-rolling steel sheets and washing liquids for removing the rolling oil after the cold rolling.
In cleaning rolling oil for cold-rolling of steel sheets and washing liquids for removing the rolling oil remaining on the surface of the cold-rolled steel sheets, a magnetic filter apparatus is used to remove magnetic particles contained in the fluids.
A typical example of a conventional magnetic filter apparatus is now explained with reference to a cross-sectional view in FIG. 1(a) and a side view in FIG. 1(b). In the drawings, reference numeral 1 denotes a container, 2 denotes a permanent magnet, 3 denotes a filter element, 4 denotes a back plate, 5 denotes a fluid inlet, and 6 denotes a fluid outlet.
A ferromagnetic component comprising a metal grid composed of iron or ferritic stainless steel such as SUS 430 is usually disposed as the magnetic filter element 3 in the interior of the container 1. At the exterior of the container 1, the permanent magnets 2 are arranged to oppose each other with the container 1 therebetween so as to generate a magnetic line of force in a direction substantially orthogonal to the flow direction of the fluid to be treated. The fluid to be treated is fed to the interior of the container 1 from the fluid inlet 5, passes through the magnetic filter element 3, and is discharged from the outlet 6. Magnetic particles such as iron particles contained in the fluid to be treated passing through the magnetic filter element 3 are magnetically attracted to the magnetic filter element 3 magnetized by the permanent magnets 2 and are separated from the fluid to be treated.
In the above-described capturing of the magnetic particles using the magnetic filter apparatus, the attractive force Fm of the filaments or metal grid constituting the filter element is expressed by the formula:
Fm="khgr"xc2x7Vxc2x7Hxc2x7(dH/dx), 
wherein
"khgr": magnetic susceptibility of the particles,
V: volume of the particles,
H: intensity of the magnetic field, and
dH/dx: magnetic gradient (spatial variation in the magnetic field.
In the above formula, "khgr" and V are inherent properties of the magnetic particles. Thus, in order to increase the attractive force Fm and improve the performance of the filter, either the magnetic field H or the magnetic gradient dH/dx must be increased. However, the magnetic gradient dH/dx is a coefficient dependent on the material and the shape of the ferromagnetic component which constitutes the filter element; accordingly, after the material and the shape of the ferromagnetic component are determined, the magnetic gradient dH/dx is regulated by the intensity of the magnetic field. Thus, the foremost requirement for improving the performance of the filter, i.e., the attractive power, is to sustain a strong magnetic field in the interior of the filter.
Hitherto, the relationship between the performance of the filter and the magnetic field has not been fully examined. Accordingly, failures such as degradation of the performance of the filter due to a diminished magnetic field in the filter have occurred frequently. As for the selection of the magnets, it is not clear what degree of strength is required from a magnet in order to achieve the desired filter performance. Moreover, because the relationship between the shape of the filter, the flow speed of the fluid to be treated, and the strength of the magnet is not clear, the filter cannot achieve the desired performance.
In other words, strong magnets do not always yield satisfactory results because of their design and specifications.
Moreover, the use of strong magnets increases the equipment cost, although some improvement can be expected.
The present invention favorably solves the above-described problems. An object of the present invention is to provide a magnetic filter apparatus of reduced size at low cost by yielding the highest possible performance from the filter in which general-purpose permanent magnets such as ferrite or neodymium magnets are used.
In order to clarify the relationship between the intensity of the magnetic field of the magnetic filter apparatus and the performance of the filter, the present inventors have conducted research on the influence of the various factors on the performance of the filter. During the course, the present inventors have succeeded in clarifying the effect of the various factors on the performance of the filter and developed a low-cost high-efficiency magnetic filter apparatus based on this finding.
That is, the present invention is a magnetic filter apparatus comprising: a container having an inlet and an outlet for fluid; a filter element comprising a ferromagnetic material disposed in the container; and permanent magnets for magnetizing the filter element, the permanent magnets being arranged to oppose each other with the container therebetween so as to generate a magnetic line of force in a direction substantially orthogonal to the moving direction of the fluid inside the container,
wherein, while regulating a filter passage time of the fluid in the range of 0.5 to 1.5 seconds, the permanent magnets are arranged so that the distance L (mm) therebetween in relation to the residual magnetic flux density B (T) of the permanent magnets satisfies the relationship:
Bxc3x97100xe2x89xa6Lxe2x89xa6Bxc3x97250 
In the present invention, the permanent magnets for magnetizing the filter element preferably have a residual magnetic flux density of 0.4 T or more.